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The Journal of Adhesion

ISSN: 0021-8464 (Print) 1545-5823 (Online) Journal homepage: http://www.tandfonline.com/loi/gadh20

Shear Strength and Durability Testing of AdhesiveBonds in Cross-laminated Timber

Karol S. Sikora, Daniel O. McPolin & Annette M. Harte

To cite this article: Karol S. Sikora, Daniel O. McPolin & Annette M. Harte (2016) Shear Strengthand Durability Testing of Adhesive Bonds in Cross-laminated Timber, The Journal of Adhesion,92:7-9, 758-777, DOI: 10.1080/00218464.2015.1094391

To link to this article: http://dx.doi.org/10.1080/00218464.2015.1094391

Accepted author version posted online: 06Oct 2015.Published online: 06 Oct 2016.

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Shear Strength and Durability Testing ofAdhesive Bonds in Cross-laminated Timber

KAROL S. SIKORADepartment of Civil Engineering, College of Engineering and Informatics, National University

of Ireland, Galway, Galway, Ireland

DANIEL O. MCPOLINSchool of Planning, Architecture and Civil Engineering, Queen’s University Belfast, Belfast, UK

ANNETTE M. HARTEDepartment of Civil Engineering, College of Engineering and Informatics, National University

of Ireland, Galway, Galway, Ireland

This paper addresses the quality of the interface- and edge-bondedjoints in layers of cross-laminated timber (CLT) panels. The shearperformance was studied to assess the suitability of two differentadhesives, polyurethane (PUR) and phenol–resorcinol–formaldehyde(PRF), and to determine the optimum clamping pressure. Since thereis no established testing procedure to determine the shear strength ofthe surface bonds between layers in a CLT panel, block shear tests ofspecimens in two different configurations were carried out, andfurther shear tests of edge-bonded specimen in two configurationswere performed. Delamination tests were performed on sampleswhich were subjected to accelerated aging to assess the durabilityof bonds in severe environmental conditions. Both tested adhesivesproduced boards with shear strength values within the edge-bondingrequirements of prEN 16351 for all manufacturing pressures.While the PUR specimens had higher shear strength values, the PRFspecimens demonstrated superior durability characteristics in thedelamination tests. It seems that the test protocol introduced in thisstudy for crosslam-bonded specimens, cut from a CLT panel, and

Received 4 August 2015; in final form 11 September 2015.Address correspondence to Karol S. Sikora, National University of Ireland, Galway, Uni-

versity Road, Galway, Ireland. E-mail: karol.sikora@nuigalway.ieColor versions of one or more of the figures in this article can be found online at www.

tandfonline.com/gadh.

The Journal of Adhesion, 92:758–777, 2016Copyright © Taylor & Francis Group, LLCISSN: 0021-8464 print/1545-5823 onlineDOI: 10.1080/00218464.2015.1094391

758

placed in the shearing tool horizontally, accurately reflects theshearing strength of glue lines in CLT.

KEYWORDS Adhesives for wood; CLT; Delamination; Durability;Engineered wood; Joint design

1. INTRODUCTION

1.1. CLT Concept

Construction materials are expected to comply with requirements reaching farbeyond a general utility market. New high-performance materials are requirednot only to be more durable and exhibit a longer life, even under severeenvironmental conditions, but having consumed less energy during their lifecycle. When compared with conventional materials, they have to be moreecologically friendly and follow sustainability trends. One promising product,satisfying the criteria of sustainability, is cross-laminated timber (CLT).

CLT is a prefabricated multilayer engineered panel wood product, withthe grain direction of consecutive layers orthogonally orientated, bonded bygluing their surfaces together with an adhesive under pressure for a specificperiod of time. This specific orientation results in increased in-plane and out-of-plane strength, rigidity, and stability. The degree of anisotropy in propertiesand the influence of natural variations, such as knots, are reduced incomparison with construction timber [1–6]. Load-bearing CLT wall and floorpanels are easily assembled on site to form multistorey buildings, improvingconstruction and project delivery time, reducing costs, and maximisingefficiency on all levels [2, 7–10].

1.2. Testing of Adhesive Bond Quality

Different standard testing procedures for determining the quality of theinterface bond between the laminations have been established, which arebased on determination of local shear strength and wood failure percentage,according to the standards such as EN 302 [11], EN 392 [12], and ASTM D 905[13]. As pointed out by Steiger et al. [14, 15], only general principles of themethods of applying shear stress to the bond line are presented in the relevantstandards. In accordance with EN 302 [11], the shear strength of adhesivebonds is determined by applying a longitudinal tensile force to a single lapjoint with close contact or thick glue lines between two rectangular woodenelements. In EN 392 [12], a cylindrical bearing is specified that is able to self-align so that the test piece can be loaded at the end-grain with a stress fielduniform in the width direction. A similar shearing tool is proposed by ASTM905 [13], however, the difference in comparison with EN 392 [12] is that thetwo blocks comprising the specimen are bonded in a staggered (lapped)

Shear Strength and Durability Testing of Adhesive Bonds in CLT 759

configuration. In all these methods, pure shear stress cannot be obtained, butthe resulting stress in the bond line is a combination of shear and normalstresses [14–17]. When the normal stresses are acting as tensile stressesperpendicular to the bond line, the recorded shear strength values rangeconsiderably below the pure shear stress level, while compression stressesperpendicular to the grain lead to an overestimation of the shear strength ofthe bond line. In order to limit this effect, Steiger et al. [14, 15] developed aprototype of a modified shear test device, which ensures a clearly definedstate of shear loading of the specimens.

Because of these limitations in the methodologies used for assessingadhesive bonds performance, it is generally accepted that no single testprocedure can provide all of the information to definitively measure bondingquality [18]. Since it is believed that many factors influence the results,including the strength of the wood, the specimen geometry, the shear tooldesign, and the rate of loading, wood failure percentage is often recorded inorder to assess the quality of adhesive bond [19]. It provides informationwhether the superior strength is in the timber or the bond, but lacksinformation on the failure behaviour [20].

In order to compare and assess the suitability of different testingprotocols for adhesive bonds, Serrano [17] modelled the adhesive layers inthe specimens in accordance with different codes, including ASTM-D 905 [13]and EN-302 [11] using a nonlinear softening, fracture mechanics model. Theresults showed that the prediction of bond line strength is highly dependenton the specimen type used and the adhesive properties. On the other hand,Davalos et al. [21] found the block-shear tests of ASTM D 905 [13] as mostsuitable for obtaining the average interface shear strengths when testing fibre-reinforced plastic (FRP)–wood bonds, where the combination of variousparameters affects measurement. The stiffness imbalance that arises from thebonding of dissimilar materials was noted as being an important issue in theshear stress distribution in other studies [22, 23]. Furthermore, when twomaterials of different stiffness are bonded together, the shear stress andtransverse normal stress in the adhesive layer are responsible for the initiationof the failure of the adhesively bonding joints near the free ends of adhesivelybonding region where the peak stresses occur [24].

In addition to the mechanical properties of adhesive, other factorsinfluencing adhesive performance such as temperature, humidity, or ageing ofthe bonds should be taken into consideration [25–27]. This was evidenced in anextensive study by Raftery et al. [28] on the hygrothermal compliance of a variety ofwood-laminating adhesiveswhen bonding FRPmaterials towood. Raftery et al. [29]also showed that with specific adhesives, cost-effective thin bond lines have thecapacity to resist severe hydrothermal stresses imposed at the FRP–wood interface.Lavisci et al. [30] examined delamination of thick joints after accelerated ageingcycles and concluded that the delamination test seemed to be effective in char-acterising the performance of the boned joint. Another factor that seemed to have

760 K. S. Sikora et al.

significant effect on the performance of adhesively bonded timber joints is occur-rence of defects. The empirical and numerical study of the influence of artificialdefects on the capacity of adhesively bonded timber joints by Grunwald et al. [31]demonstrated that joints with a 50% defect area still achieved a capacity of 70% ofthat of defect-free joints.

1.3. CLT Delamination Testing

The provisional European Standard EN 16351:2013 [32] is the first Europeancode strictly dedicated to CLT that sets out provisions regarding theperformance characteristics of CLT for use in buildings and bridges. Accordingto prEN 16351 [32], the resistance of edge bonding has to be controlled bymeans of block shear tests according to EN 392 [12]. For controlling theadhesion or the resistance against fractures in the bond line, specimens ofdefined geometry have to be exposed to a specific series of climaticconditions and afterwards the delamination of their bond lines has to bedetermined (more details in Section 2.2).

In accordance with Canadian [33] and US [34] CLT handbooks, woodfailure results from block shear specimens tested under vacuum–pressure–dryconditions can be used to assess the bond quality. It is considered thatdry wood failures lacked consistency and should not be considered as areasonable criterion in assessing the bond quality of CLT panels. Only thevacuum–pressure–dry wood failures showed consistency in assessing thebond quality of CLT panels [35]. In addition to the influence of timber moisturecontent and temperature, factors such as distortion and wane have a negativeinfluence on bonding strength due to their effect on the bond line geometry.Therefore, in accordance to ANSI/APA PRG 320-2012 [36], an “effectivebonding area,” defined as the proportion of the lamination wide face averagedover its width that is able to form a close bond upon application of pressure,of 80% is required.

In order to clarify the consequences of the interacting parametersbonding pressure and spreading rate on CLT production, a comprehensiveresearch project was conducted [3, 37]. Two types of one-componentpolyurethane (1K-PUR) adhesives, three bonding pressures of 0.1, 0.3, and0.6 N/mm2, and various spreading rates were investigated. Additionally, theeffect of cyclic climatic variations (20°C/90% RH and 30°C/40% RH; number ofcycles: 0, 10, 21, 25) on the properties of bonding was also analysed. Thebonding properties were investigated by means of rolling shear tests on wholeCLT elements in bending according to EN 408 [38], block (rolling) shear testson the single glue line according to EN 392 [12], and delamination testsaccording to EN 391 [39]. The investigated bonding pressures were foundto be sufficient to realise adequate bond qualities provided the thicknessvariations between boards of the same CLT layer was kept low. It was

Shear Strength and Durability Testing of Adhesive Bonds in CLT 761

found that parameters like warp or twist of the board material showed nearlyno or at least negligible effects on surface bonding. Further, a positiverelationship between bonding pressure and shear strength was observed incases where the applied spreading rate was lower than that recommended bythe manufacturer or the deviations in thickness were too high.

1.4. Adhesives Systems for CLT

Generally, adhesives are grouped according to their chemistry [25, 40].However, Frihart [41] proposed to consider not only the chemical but alsothe mechanical response of adhesives and therefore suggested to differentiatebetween two main groups: in situ polymerised and pre-polymerisedadhesives. The in situ polymerised adhesives contain relatively rigid, highlycrosslinked polymers such as urea–formaldehyde (UF), melamine–formalde-hyde (MF), melamine–urea–formaldehyde (MUF), phenol–formaldehyde(PF), phenol–resorcinol–formaldehyde (PRF), but also polymeric methylene-diphenyl-diisocyanate (pMDI), whereas the second group includes flexiblepolymers such as PUR and polyvinyl acetate (PVAc). These two groups differsignificantly in their ability to distribute moisture-induced stress in an adhesivebond resulting in different failure mechanisms.

The adhesive systems which are allowed for use in CLT productionaccording to prEN 16351 [31], and the Canadian [33] and US [34] CLThandbooks are

– phenoplast- and aminoplast adhesives: these include primarily MUF andPRF adhesives,

– one-component polyurethane adhesives (1K-PUR);– emulsion–polymer–isocyanate adhesive (EPI).

Typical characteristics of these adhesives are presented in Table 1. It shouldbe noted that while Table 1 gives recommended values for wood moisturecontent, application rate, applied pressure, and assembly and pressing times,in practice specific manufacturers’ requirements must be followed.

PRF is a popular adhesive for structural use (commonly used for glulammanufacturing), which is the cheapest (per kilogram) among such adhesive sys-tems. However, PRF requires a higher spreading rate than PUR(approximately three times) and EPI, and much longer pressing time than EPIand PUR. PRF is dark brown, which may be an issue in terms of aesthetic quality,and contains formaldehyde, whereas EPI and PUR are light-coloured and formal-dehyde-free. Due to the chemical reaction with water, PUR produces slight foam-ing during hardening. PRF, EPI, and PUR are in principle suitable for bonding offinger joints as well as edge and surface bonding, however EPI, according to prEN16351 [32], is not allowed for large finger joints.

762 K. S. Sikora et al.

1.5. Objectives of the Present Study

In order to address the quality of the interface bonds in CLT, it has beenintended to:

– assess the suitability of different adhesives and to determine the optimumclamping pressure;

– assess the durability of adhesive bonds;– make recommendations on suitable testing protocol for adhesive bonds inCLT.

2. MATERIALS AND METHODS

In order to realise the objectives of this study, a research program consist-ing of shear and delamination tests was carried out. Further, for sheartesting, specimens of two geometries were manufactured, one group ofspecimens, edge bonded in accordance with prEN 16351 [32], and anothergroup, faced bonded, cut from manufactured CLT panels. Loadings duringshear testing were applied in two different directions for each specimengroup, as shown in Fig. 1 (abbreviations for each specimen configurationare also presented).

Specimens for delamination tests were also cut from CLT panels. Thedelamination tests followed procedures outlined in prEN 16351 [32]. Twotypes of adhesives, PUR and PRF, using four different manufacturingpressures, were used for specimen preparation during the course of thisstudy.

TABLE 1 Typical Characteristics of Adhesives for CLT Manufacturing [34]

Item

Adhesive

PRF EPI PUR

Cured adhesive colour Dark Light LightComponent Liquid, two

componentsLiquid, twocomponents

Liquid, singlecomponent

Solids content (%) 50 43 100Wood moisture content (%) 6–15 6–15 >8Target application rate(single spread) (g/m2)

375–400 275–325 100–180

Assembly time (min) 40 20 45Pressing time (min) 420–540 60 120Applied pressure (N/mm2) 0.8 0.8 0.8–1.4Cost ($/kg) 4.4 7.7 10.6

Shear Strength and Durability Testing of Adhesive Bonds in CLT 763

2.1. Materials

2.1.1. TIMBER

In order to ensure a uniform moisture content of 12% (measured by HandheldMoisture Meter GE Protimeter BLD5602 Timbermaster) in the specimens duringthe testing, boards of C16 Irish Sitka spruce (Picea sitchensis) were stored in aconditioning chamber (65% ± 5% RH, 20° ± 2°C) for 3 months before specimenpreparation. Subsequently, all sides of the boards were planed by a specialisedcompany to cross-sectional dimensions of 94 mm × 30 mm. A tight tolerance onthe lamination thickness is required for the production of CLT due to the thinbond lines used. Because of this, thickness measurements were taken on theboards immediately after planing to determine whether the required tolerance of0.1 mmwas achieved. The boards that failed to meet the required tolerance wereexcluded when the test specimens were manufactured.

2.1.2. ADHESIVES

A 1K-PUR adhesive (PURBOND HB S309, Purbond AG, Sempach, Switzerland)and a two-component PRF adhesive (Prefere 4050 Mwith hardener Prefere 5750,Dynea UK, Flintshire, UK, using a ratio of 1:1), formulated for the manufacture ofengineered wood products systems, were used to bond the edges of the sheartest specimens. The reasons for such selection are related to extremes in values ofrelevant factors between these two systems: application rate, pressing time, andcosts. In addition, their structural performance is considered to be superior to EPI.

2.2. Methodology

2.2.1. SPECIMEN PREPARATION

The adhesive systems were applied on one of the bonded surfaces at the rateof 160 g/m2 for PUR, and on both surfaces at the rate of 400 g/m2 (200 g/m2

Shear

specimens

Edge bonded

End-grain (E)Perpendicular

to grain (P)

Face bonded

Crosslam

vertical (V)

Crosslam

horizontal (H)

FIGURE 1 Schema of specimen configurations for shear tests.

764 K. S. Sikora et al.

on each surface of glue line) for PRF, as recommended by the adhesivemanufacturers. Four different values of pressure, namely 0.4, 0.6, 0.8,and 1.0, were applied by a compressive testing machine for 120 min for thePUR-bonded specimens and for 16 hr for the PRF-bonded specimens. Pressingtime is a function of temperature and, as the ambient laboratory temperaturewas approximately 17°C for the PRF-bonded specimens, the selected pressingtime was to ensure compliance with the manufacturer’s recommendedminimum for cold bonding (15 hr for 15°C) [42]. The manufacturersrecommend applying pressure from 0.6 to 1.0 N/mm2 for softwoods, forboth adhesives. These were addressed in this study and additionallysamples prepared using lower pressure, 0.4 N/mm2, were tested to assessthis lower pressure potential usage that may facilitate CLT production. Afterreconditioning (65% ± 5% RH, 20° ± 2°C), test specimens were cut to size.

The two sets of specimens, which were edge bonded, had bonded areasof dimensions 30 mm thick and 50 mm wide, in accordance with prEN16351:2013 [32]. In addition, solid wood specimens, without glue lines, ofthe same cross-sectional dimensions were prepared.

In order to prepare specimens for the shear tests of crosslam-bondedelements (specimens bonded orthogonally) and the delamination tests,sample CLT panels of 90 mm (three layers of 30 mm) thickness weremanufactured. Panels were face-bonded only; there were no edge bonds inthese CLT panels. After reconditioning (for minimum 2 weeks to 12% moisturecontent), specimens for the shear tests of crosslam-bonded elements and forthe delamination tests of glue lines between layers were cut from these panels.These specimens had cross-sectional dimensions of 30 mm × 50 mm same asfor edge-bonded specimens. Figure 2 presents schemas of the shear testspecimens for end-grain (a) and perpendicular to grain loading directions(b), and shear test specimens for crosslam bonded elements vertical (c) andhorizontal (d) loading directions.

Table 2 presents the number of shear test specimens for the differentbonding pressures, adhesives, and test configurations.

The delamination tests were carried out on 10 specimens of 100 mm ×100 mm × 90 mm for each adhesive type and manufacturing pressure. Thenumber of specimens for shear and delamination tests is in accordance with

)d()c()b()a(

30

50

50

30

30

50

50

30

NTS

FIGURE 2 Shear test specimens for: (a) end-grain, (b) perpendicular to grain loading, andcrosslam bonded elements vertically (c) and horizontally (d) loaded (dimensions inmillimetres).

Shear Strength and Durability Testing of Adhesive Bonds in CLT 765

recommendations of “Factory production control for cross laminated timberproducts” from prEN 16351:2013 [32].

2.2.2. SHEAR TESTING

The shear tests were carried out by applying a compressive force using a shearingtool in accordance with EN 392 [12]. The cylindrical bearing was able to self-alignso that the test piece could load at the end-grain and perpendicular to grain with astress field uniform in the width direction. The EN 392 [12] standard requiresloading tested specimens at the end-grain. However, since in CLT panels thewood grain of each layer are orientated perpendicular to wood grains of layerswith which it is in contact, the shear stresses occur in different planes. Because ofthis, tests were carried out with specimens loaded perpendicular to grain, and forthe crosslam specimens. Loads were applied in the vertical and horizontal direc-tions. Loading was applied under displacement control at a rate of 3 mm/min,ensuring failure after no less than 20 s, which is in accordancewith EN 392 [12] andstudies by Steiger et al. [14, 15]. Just after the shearing tests, 50-mm long portionswere cut from each specimen, and weighted in order to determine the density.

For the purpose of the shear testing analyses, Student’s t-test was carriedout for comparison of shear strengths results for different manufacturingpressures. As a matter of good scientific practice, a significance level of 5%was chosen for a two-tailed test for two-sample unequal variance.

2.2.3. DELAMINATION TESTING

The test programme and procedure were in accordance with Annex C of prEN16351:2013 [32]. Test pieces for the glue line delamination tests were placedin a pressure vessel and submerged in water at a temperature of about 15°C.Then a vacuum of about 80 kPa was drawn and held for 30 min.Subsequently, the vacuum was released and pressure of about 550 kPa wasapplied for 2 hr. Later, the test pieces were dried for a period of approximately15 hr in a circulating oven at a temperature of 70° ± 5°C. After removal fromthe oven, the delaminated length for each of the two glue lines was measuredaround the perimeter of the specimen. The lower value of the wood fibres

TABLE 2 Numbers of Shear Tests

Bonding pressure (N/mm2) 0.4 0.6 0.8 1.0Solid wood

(unglued) (SW)Adhesive type PUR PRF PUR PRF PUR PRF PUR PRF

End-grain (E) 36 18 36 18 36 18 36 18 36Perpendicular to grain (P) 36 18 36 18 60 18 36 18 36Crosslam vertical (V) 16 34 16 32 17 30 16 22 –

Crosslam horizontal (H) 16 32 16 32 17 34 16 22 –

766 K. S. Sikora et al.

failure percentages from the two glue lines, FFmin, and the sum of the two splitareas, FFtot, were recorded.

3. RESULTS

3.1. Shear Tests

The shear strength fv was determined for every tested glue line and wascalculated in accordance with the following formula:

fv ¼ FuA; (1)

where Fu is the ultimate load (in Newtons) and A; the sheared area (in squaremillimetres).

Figure 3 presents the mean (M), 5-percentile (5%), and standard deviations(SD) of shear strengths for samples manufactured with different pressures andconfigurations, and for solid wood (SW) specimens.

Difference between test methods led to large differences in results. Thevalues for end-grain-loaded specimens on average were at least threetimes higher than for other testing configurations. The differences betweenedge-bonded specimens loaded perpendicular to grain and crosslamspecimens were less pronounced. The 5-percentile shear strengths for gluelines loaded at end-grain were very consistent for PUR adhesive type, and werebetween 7.3 N/mm2 (manufactured with pressure of 0.4, 0.8, 1.0 N/mm2) and 7.6N/mm2 (manufactured with pressure of 0.6 N/mm2). In addition, these resultswere in line with the result for solid wood specimens, which was 7.4 N/mm2. For

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Sh

ea

r s

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th

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/m

m2

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FIGURE 3 Shear strength values*.

*Abbreviations on horizontal axis represent: number – manufacturing pressure (N/mm2), letter – specimenconfiguration (e.g., 0.4E – specimen manufactured using 0.4 N/mm2, loaded end-grain during shear test).

Shear Strength and Durability Testing of Adhesive Bonds in CLT 767

the equivalent specimens bonded using the PRF adhesive system, the 5-percen-tile shear strength values varied more and were between 6.4 N/mm2 for 0.4 N/mm2 and 8.4 N/mm2 for 1.0 N/mm2 manufacturing pressures. Standard deviationvalues were around 0.52 N/mm2 for all edge-bonded, end-grain-loaded speci-mens, 0.36 N/mm2 for perpendicular to grain loaded, 0.23 N/mm2 for crosslamspecimens, vertically placed, and 0.18 N/mm2 for crosslam specimens, horizon-tally placed in shear block tool. A mean density of 427.12 kg/m3 with standarddeviation of 42.51 kg/m3 was obtained for all tested samples.

3.2. Delamination of Glue Lines

The total delamination Delamtot of each test piece was calculated usingEquation (2):

Delamtot ¼ 100ltot;delamltot;glueline

%ð Þ; (2)

where ltot,delam is the total delamination length (in millimetres) andand ltot,glueline,the sum of the perimeters of all glue lines in a delamination specimen (inmillimetres).

The maximum delamination Delammax of a single glue line in each testpiece was calculated from Equation (3):

Delammax ¼ 100lmax;delam

lglueline%ð Þ; (3)

where lmax,delam is the maximum delamination length (in millimetres) andlglueline, the perimeter of one glue line in a delamination specimen (inmillimetres).

The delamination requirement in prEN16351 [31] can be satisfied in oneof two ways:

– Condition (1): Delamtot ≤ 10% and Delammax ≤ 40% for all samples

or

– Condition (2): If condition (1) is not satisfied, the wood failure percentagefor each split glued area, FF, must be ≥ 50% and for the sum of the two splitareas must be ≥ 70%.

In Fig. 4 median values are presented from the following results for specimensmanufactured using different pressures: total and maximum delamination, andthe lower of the wood failure percentages from the two glue lines and the sumof the two split areas. In addition, maximum values of Delamtot and Delammax,

768 K. S. Sikora et al.

and minimum of FFmin and FFtot of all specimens for different manufacturingpressures are presented.

Delamination condition (1) of prEN 16351 [32] was not satisfied in any of thespecimens, but condition (2) was fulfilled for specimens manufactured using PURadhesive with 0.8 N/mm2 pressure and PRF system with 1.0 N/mm2 pressure.

4. DISCUSSION

4.1. Bonding Strength

4.1.1. THE EFFECT OF MANUFACTURING PRESSURE

The shear tests results give an indication that the lowest pressure of 0.4 N/mm2 applied during manufacturing of the specimens is sufficient for Irish Sitkaspruce in terms of the prEN 16351:2013 [31] shear strength requirements forboth adhesive systems despite the manufacturers’ minimum requirementof 0.6 N/mm2. Five-percentile shear strength values for different testconfigurations manufactured with different pressures and adhesive systemsof PUR and PRF are compared in Fig. 5.

Student’s t-test statistical result for the specimens manufactured atdifferent pressures compared with a reference pressure of 1.0 N/mm2 isgiven in Table 3. From this table, it can be seen that for PUR bondedspecimens, the processing pressure does not result in significantly differentshear strength results except in the case of edge-bonded specimens producedusing a pressure of 0.4 N/mm2. For the case of PRF crosslam-bondedspecimens, clamping pressure has no significant effect on shear strengthperformance (a minor deviation was recorded for PRF H for 0.8 N/mm2).

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D-mtot D-mmax FFmin FFtot D-mtot D-mmax FFmin FFtot D-mtot D-mmax FFmin FFtot D-mtot D-mmax FFmin FFtot

18.06.04.0

De

lam

ina

tio

n/W

oo

d F

ibre

Fa

ilu

re

PUR PRF

Manufacturing pressure [N//mm2]:

FIGURE 4 Delamination tests results†.†D-mtot is the total delamination Delamtot, D-mmax is the maximum delamination Delammax.

Shear Strength and Durability Testing of Adhesive Bonds in CLT 769

However, for edge-bonded specimens loaded at the end-grain, there is asignificant difference when comparing a clamping pressure of 1.0 N/mm2

with all lower pressures. When compared with the shear strength of solidwood specimens, PRF E specimens manufactured using a pressure of 1.0 N/mm2 had slightly higher but not significantly different shear strength (Table 4).However, significant differences were found for PRF P and PUR E and PUR Pspecimens, as shown in Table 4.

Furthermore, the recordings of wood failure percentages confirmedthe observations by Steiger et al. [14, 15] that for specimens loaded at theend-grain, the values for PUR type adhesives are generally very high andexhibit a small variation. Figure 6 presents median wood failure percentage

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0.4 0.6 0.8 1.0 SW

Sh

ea

r s

tre

ng

th

[N

/m

m2]

Manufacturing pressure [N/mm2]

PUR E PRF E PUR P PRF P

PUR V PRF V PUR H PRF H

FIGURE 5 Five-percentile shear strength values for different test configurations.

TABLE 3 Student’s t-Test p-Values for Comparison of Shear Tests Results for ManufacturingPressure of 1.0 N/mm2 with Lower Manufacturing Pressure for Specimens Produced with PURand PRF Adhesives in Different Configurations

Bonding pressure (N/mm2) Adhesivetype and test configuration 0.4 0.6 0.8

PUR E 0.0022 0.6956 0.6737PRF E 0.0000 0.0000 0.0000PUR P 0.0007 0.5111 0.0563PRF P 0.2820 0.7588 0.0667PUR V 0.1302 0.6126 0.1154PRF V 0.9875 0.4426 0.9932PUR H 0.0656 0.2599 0.3789PRF H 0.6493 0.1923 0.0376

770 K. S. Sikora et al.

values for different configurations of specimens manufactured using PUR andPRF adhesives with different pressures.

Generally, lower wood failure percentages were observed for specimensmanufactured with PRF than for corresponding specimens with PUR, which isin line with the effect of PRF on shear strength. The lower results for thepressure of 0.6 N/mm2 might be associated with variability within timber.

4.1.2. THE EFFECT OF ADHESIVE TYPE

Comparison of results between PUR and PRF systems for different clampingpressures and testing configurations showed insignificant differences incorresponding samples. The ratios of PUR to PRF 5-percentile shear strengthsdiffer in most cases by less than 10% (the exception is 22% for crosslam

TABLE 4 Student’s t-Test p-Values for Comparison of Shear Tests Results for Solid WoodSpecimens with Glue Lines Manufactured with 1.0 N/mm2 Pressure for Specimens Producedwith PUR and PRF Adhesives Loaded End-grain and Perpendicular to Grains

Bonding pressure (N/mm2) Adhesivetype and test configuration 1.0

PUR E 0.0141PRF E 0.1285PUR P 0.0000PRF P 0.0000

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

1.00.80.60.4

Wo

od

fa

ilu

re

%

Manufacturing pressure [N/mm2]

PUR E PRF E PUR P PRF P

PUR V PRF V PUR H PRF H

FIGURE 6 Median wood failure percentage values for different configurations of specimensmanufactured with different pressures.

Shear Strength and Durability Testing of Adhesive Bonds in CLT 771

samples manufactured using 0.8 N/mm2 pressure and loaded vertically), aspresented in Table 5.

There is no general consistency in these results; however, the ratios forcrosslam specimens loaded horizontally are very close to 1.00, giving anindication that adhesive type has no effect on structural bonding performance,which is confirmed by Student’s t-test. It is very likely that slight differences inthe ratios are determined by wood performance.

4.1.3. EFFECT OF TEST CONFIGURATION

For edge-bonded specimens, the 5-percentile shear strength values ofspecimens loaded through the end-grain are 3.5 times of those loadedperpendicular to grain, which is shown in Fig. 7.

The corresponding ratio for solid wood specimens loaded at the end-grain to those loaded perpendicular to grain is 2.8. When values of specimensloaded through the end-grain are compared with crosslam specimens, ratiosvary between 3 and 6, depending on manufacturing pressure. It should benoted that the strength ratio for crosslam specimens loaded vertically to thoseloaded horizontally varied between 0.64 and 1.00. It is likely that this isassociated with more tilting of the V-type specimens during testing, as these

TABLE 5 PUR to PRF Ratio of 5-Percentile Shear Strength Values and Student’s t-Test p-Values(in Parentheses) for Different Manufacturing Pressures and Test Configurations

Bonding pressure (N/mm2)Test configuration 0.4 0.6 0.8 1.0

E 1.13 (0.0001) 1.07 (0.0000) 1.08 (0.0000) 0.87 (0.0354)P 0.92 (0.0532) 1.04 (0.2914) 1.01 (0.0345) 0.92 (0.0331)V 0.92 (0.6788) 0.92 (0.5347) 1.22 (0.7025) 1.13 (0.0695)H 0.98 (0.0383) 0.96 (0.8608) 0.98 (0.1364) 1.01 (0.0965)

0

1

2

3

4

5

6

0.4 0.6 0.8 1.0 SW

Manufacturing pressure [N//mm2]:

FIGURE 7 Ratios of 5-percentile shear strength values for different manufacturing pressures.

772 K. S. Sikora et al.

specimens were more slender than the H specimens. Such a phenomenonwas noticed by Steiger et al. [14, 15]. Therefore, it seems that these tests oncrosslam-bonded specimens placed in the shearing tool horizontally mostaccurately reflect the shearing strength of glue lines in CLT. In addition, theresults for the H configuration were slightly more consistent than for the Vconfiguration, as shown by the standard deviation values.

4.2. Bonding Pressure and Adhesive Type Effect on Durability

Although delamination results varied significantly between the test pieces, it isvery likely that the mechanism resulting in the delamination of glue lines wasthe same for all specimens. In vast majority of cases, delamination occurred ina single glue line on one side. Since the vacuum–pressure–soak cycle resultedin swelling, which was much higher in the tangential and radial directionsthan in the longitudinal direction for the timber, it induced significant internalshear stresses between the bonded surfaces. Furthermore, since the CLT layerswere not edge-bonded, small gaps are present between adjacent boards ineach layer. Delamination always occurred at the shortest edge board, as seenin Fig. 8(c).

It seems that median values are the most realistic measure to assess theresults of the delamination tests, since the extreme results are excluded, whichmay otherwise skew the overall result. Therefore, the median values of totaland maximum delaminations, and total and maximum wood fibre failuresof split surfaces are shown in Fig. 9. Although, there are no noticeabledifferences between the total and maximum delimitation results for PURand PRF adhesive systems, it was observed that the highest manufacturingpressure of 1.0 N/mm2 provided the most durable bonds. This phenomenonwas slightly more pronounced for PUR.

On the other hand, the trends of wood fibre failure percentages, total andminimum, for PUR and PRF adhesive systems vary considerably. High values forPRF, above 80% for minimum wood fibre failure for all manufacturing pres-sures, indicate very good durability performance of PRF glue lines. For PUR,minimum wood fibre failures were noticeably low for panels assembled withpressures of 0.4 and 0.6 N/mm2, suggesting poor durability. However, for

)c()b()a(

FIGURE 8 Specimen for delamination test (a) before and after vacuum-pressure cycle (b, c).

Shear Strength and Durability Testing of Adhesive Bonds in CLT 773

specimens manufactured with higher pressures, values of wood fibre failureswere much higher, up to 100% (minimum and total), which pointed out thesubstantial effect of bonding pressure on durability of specimens bonded usingPUR adhesive. Such phenomenon might be associated with deeper glue pene-tration from bonded surfaces inside wood for specimens manufactured withhigher pressure. For the lower manufacturing pressures when adhesive pene-tration is shallower, resulting in thicker bonding layer, and as a consequence,the larger area of adhesive surface on the edges of a sample is directly exposedto water. Therefore, this effect of increased durability for higher bondingpressure is much more pronounced for PUR than PRF, because PUR reactswith moisture and PUR is more vulnerable to water action than PRF.

5. CONCLUSIONS

Based on the investigations presented in this study the following conclusions canbe formulated:

– Both adhesives, PUR and PRF, produced boards with shear strengthvalues within the requirements of prEN 16351 for all manufacturing pres-sures. The lowest pressure of 0.4 N/mm2 applied during manufacturing of

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

1.00.80.60.4

De

lam

ina

tio

n/W

oo

d F

ibre

Fa

ilu

re

Manufacturing pressure [N/mm2]

PUR Delamtot PRF Delamtot PUR Delammax PRF Delammax

PUR FFmin PRF FFmin PUR FFtot PRF FFtot

FIGURE 9 Median delamination and wood fibre failures values for specimens manufacturedwith different pressures.

774 K. S. Sikora et al.

the specimens is sufficient for Irish Sitka spruce in terms of the prEN16351:2013 shear strength requirements for edge bonding.

– While the PUR specimens had higher shear strength values than PRF specimenswhen the manufacturing pressure was up to 0.8 N/mm2, the durabilitycharacteristics in the delamination tests were unsatisfactory for PUR specimensmanufactured with pressures below 0.8 N/mm2. The PRF specimensdemonstrated superior durability characteristics in the delamination tests,providing satisfactory results for the pressure of 0.4 N/mm2 applied duringmanufacturing of the specimens. Furthermore, it was established that the widthsof the narrowest timber elements in CLT test piece determine the depth ofdelamination.

– Annex D of prEN 16351:2013 specifies that loading of the parallel bondedspecimens should be applied through the end-grain for testing edge bonds;however, there is lack of testing protocol, in this standard, for calculatingshear strength of surface bonds in CLT panels. It seems that the test protocolintroduced in this study for crosslam-bonded specimens, cut from CLTpanel, and placed in the shearing tool horizontally, accurately reflects toshearing strength of glue lines in CLT. Due to the relative simplicity of thismethod, it may be considered as an indicator of shear strength of bondsbetween the layers comprising CLT.

ACKNOWLEDGEMENT

This work has been carried out as part of the project entitled “Innovation inIrish Timber Usage” (project ref. 11/C/207).

FUNDING

This study is funded by the Department of Agriculture, Food and the Marine ofthe Republic of Ireland under the FIRM/RSF/COFORD scheme.

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Shear Strength and Durability Testing of Adhesive Bonds in CLT 775

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Shear Strength and Durability Testing of Adhesive Bonds in CLT 777